Sedimentation field flow fractionation is a versatile technique for the high resolution separation of a wide variety of particulates suspended in a fluid medium. The particulates include macromolecules in the 10.sup.5 to the 10.sup.13 molecular weight (0.001 to 1 .mu.m) range, colloids, particles, micelles, organelles and the like. The technique is more clearly described and more explicitly described in U.S. Pat. No. 3,449,938, issued June 17, 1969 to John C. Giddings and U.S. Pat. No. 3,523,610, issued Aug. 11, 1970 to Edward M. Purcell and Howard C. Berg.
Field flow fractionation is the result of the differential migration rate of components in a carrier or mobile phase in a manner similar to that experienced in chromatography. However, in field flow fractionation there is no separate stationary phase as is in the case of chromatography. Sample retention is caused by the redistribution of sample components between the fast to the slow moving strata within the mobile phase. Thus, particulates elute more slowly than the solvent front. Typically a field flow fractionation channel consisting of two closely spaced parallel surfaces is used. A mobile phase is caused to flow continuously through the gap between the surfaces. Because of the narrowness of this gap or channel (typically 0.025 centimeters (cm)), the mobile phase flow is laminar with a characteristic parabolic velocity profile. The flow velocity is the highest at the middle of the channel and the lowest near the two channel surfaces.
An external force field of some type (the force fields include gravitational, thermal, electrical, fluid cross-flow and others described variously by Giddings and Berg and Purcell), is applied transversely (perpendicular) to the channel surfaces or walls. This force field pushes the sample components in the direction of the slower moving liquid strata near the outer wall. The buildup of sample concentration near the wall, however, is resisted by the normal diffusion of the particulates in a direction opposite to the force field. This results in a dynamic layer of component particles, each component with an exponential-concentration profile. The extent of retention is determined by the time-average position of the particulates within the concentration profile which is a function of the balance between the applied field strength and the opposing tendency of particles to diffuse.
In sedimentation field flow fractionation, use is made of a centrifuge to establish the force field required for the separation. For this purpose, a long, thin annular belt-like channel is made to rotate within a centrifuge. The resultant centrifugal force causes components of higher density than the mobile phase to settle toward the outer wall of the channel. For equal particle density, because their of higher diffusion rate, smaller particulates will accumulate into a thicker layer against the outer wall than will larger particles. On the average, therefore, larger particulates are forced closer to the outer wall.
If now the fluid medium, which may be termed a mobile phase or solvent, is fed continuously in one end of the channel, it carries the sample components through the channel for later detection at the outlet of the channel. Because of the shape of the laminar velocity profile within the channel and the placement of particulates in that profile, solvent flow causes smaller particulates to elute first, followed by a continuous elution of sample components in the order of ascending particulate mass.
A rotating seal is used to couple the fluid medium or solvents in and out of the flow channel. Unfortunately, conventional rotating seals that have been designed for use in continuous flow type centrifuges have proven inadequate. The problems encountered with such seals are caused primarily by the fact that with the annular channels used in field flow fractionation, relatively large weights are placed at large radial distances. Under these conditions it is relatively difficult to achieve the degree of balance needed to reduce vibrations to an acceptable level to allow the use of conventional rotating seals.
This vibration problem is exacerbated when split ring type channels (described in an application Ser. No. 125,855, filed Feb. 29, 1980, entitled "Rotor for Sedimentation Field Flow Fractionation," by John Wallace Grant) are used since split ring channels tend to create some weight inbalance. With such vibrations, leakage in the seal face occurs and also seal wear becomes significant. The result is a short seal life and a sometimes severe limitation on rotor speeds that can be used. One solution to this problem is to mount the rotating seal as an independently floating face seal. While workable, such solutions have proven expensive and generally fail to produce what may be deemed a practical working system.
Thin metal tubes have been tried but also have proven unsatisfactory since they become subject to a "set" with the tubes becoming crimped resulting in even larger vibrations imposed upon the connected rotating seal assembly. This results in unacceptable seal leaks and relatively short seal lifetimes.